Chip-scale fT/cm/Hz1/2 optical magnetic gradiometry for gradient magnetoencephalography imaging at room temperature Magnetoencephalography (MEG) provides a direct and non-invasive modality to brain electrophysiological activity, with fundamental measurement principles and signal dynamics highly distinct yet complementary to electroencephalography. Current leading instrumentations such as a superconducting interference device enables remarkable few fT sensitivities, significant for imaging of postsynaptic potentials and about 10(7) smaller than Earth?s static magnetic field. Operating the superconducting sensor, however, requires liquid helium cryogenics, large-sized insulating dewar tanks, and heavily-field-shielded expensive environments, resulting in specialized imaging centers in the country and hardly portable for frontline diagnostics or wide-spread hospital usage. Objective: here we propose to demonstrate a chip-scale room-temperature magnetic gradiometer with similar sensitivities in unshielded ambient environments, enabled by our gradient approach and our recent laser measurements at the thermodynamical limits. Our gradiometer is based on a laser-driven silicon optomechanical resonant oscillator, combined with static Lorentz-force magnetic field sensing, for optical readout at 1 fT/cm/Hz1/2 sensitivities and 5 fT/cm accuracies at room temperature. Our precise and accurate sensor intrinsically measures the magnetic gradient and has similar performance metrics of the magnetic field energy resolution per unit bandwidth, compared to state-of-the-art superconducting interference devices and optically-pumped spin-exchange-relaxation-free atomic magnetometers. In this R21 exploratory grant, we will demonstrate the ?first light? measurements on the gradiometer, with the three Specific Aims: (1) demonstrating a dual-loop fT/cm/Hz1/2 magnetic gradiometer on- chip and at room temperature; (2) demonstrating gradiometer co-localization with a total field magnetometer, along with RF signal processing; and (3) validation of gradiometer in simulated sources and MEG testbeds.